1. Field of the Invention
This invention relates to a semiconductor pressure sensor for detecting a pressure and an exhaust system including the same.
2. Description of the Prior Art
In an exhaust system for a diesel engine, a particulate component removing device called as a diesel particulate filter (DPF) is used for removing particulate component in the exhaust gas from the diesel engine.
FIG. 4 shows such a prior art exhaust system. The DPF 120 is provided at an intermediate portion of an exhaust tube 122 between an engine 121 and a muffler to catch black smoke particulate components in the exhaust gas from the engine through an exhaust manifold 123 to prevent the black smoke from being exhausted from the muffler.
During the operation of the DPF 120, black smoke components accumulate at the filter in the DPF 120. Thus, the accumulated black smoke components clog the filter. The accumulated black smoke components are periodically burned with a heater for cleaning for example.
The clogging of the filter is detected based on the pressure difference between the front and the rear of the filter or the pressure difference between the front of the filter and the atmospheric pressure. In this prior art, a semiconductor pressure sensor 124 is provided between the DPF 120 and the engine 121 to detect the pressure at the front of the filter.
However, the detection signal from the semiconductor pressure sensor 124 includes a pulsate exhaust pressure component as shown in FIG. 5.
The semiconductor pressure sensor 124 has a pressure response of several milliseconds. On the other hand, the exhaust gas pressure pulsates at a cycle of about 15 ms. Accordingly, the detection signal of the semiconductor pressure sensor 124 includes the pulsate exhaust gas component.
The curve shown in FIG. 5 represents the pressure pulsation of the exhaust gas in the case that a motor vehicle with a diesel turbo engine having a piston displacement of 3000 cc with intercooler travels on a slope.
Here, if it is assumed that the detection range of the semiconductor pressure sensor 124 is determined to include the peak to peak of the pulsation, the detection range would become large because the pressure variation due to clogging of the filter to be detected is about 5 kPa but the peak to peak of the pulsation is about 40 kPa. Accordingly, the resolution of the semiconductor pressure sensor 24 is reduced. This decreases the accuracy in detection pressure variation due to the clogging. Thus, the pulsating component should be removed from the detection signal of the semiconductor pressure sensor 124.
FIGS. 6A and 6B show prior art pulsation component removing structures. In FIG. 6A, a capacitive portion is provided in the tube 125 for introducing the pressure to the semiconductor pressure sensor 124, wherein the capacitive portion 126 has a larger diameter than the tube 125. On the other hand, in FIG. 6B, a recess portion 127 is provided in the tube 125 for introducing the pressure to the semiconductor pressure sensor 124, wherein the recess portion 127 has a smaller diameter than the tube 125. These structures retards pressure propagation to the semiconductor pressure sensor 124 to remove the pulsating components.
However, standardization of these structures are difficult because the optimum forms of the capacitive portion 126 and the recess portion 127 are dependent on every form of the tube 125.
On the other hand, Japanese utility model application provisional publication No. 62-160342 discloses a full bridge circuit with a low pass filter. FIG. 7 shows this prior art full bridge circuit in a pressure sensor including semiconductor piezoelectric resistive elements R101, R102, R103, and R104. Input terminals A0 and B0 of this full bridge circuit are connected to a voltage supply 32. The output terminals C0 and D0 are connected to a non-inverting input and an inverting input of an operational amplifier 31, respectively.
When a pressure is applied to this semiconductor pressure sensor, resistances of the semiconductor piezoelectric resistive elements R101 to R104 vary. This generates a voltage difference between the output terminals C0 and D0 which are connected to one ends of capacitors C10 and C20, respectively. The other ends of the capacitors C10 and C20 are connected to the ground.
Therefore, the capacitors C10 and C20 and semiconductor piezoelectric resistive elements R101, R102, R103, and R104 form low pass filters.
These low pass filters retard the response in the full bridge circuit. Thus, this full bridge circuit in the pressure sensor 124 can remove the pulsate components.
This full bridge circuit has two capacitors having a relatively large capacitances, so that the chip size including the semiconductor pressure sensor becomes relatively large.
The aim of the present invention is to provide a superior semiconductor pressure sensor and a superior exhaust system including the semiconductor pressure sensor.
According to the present invention, a first aspect of the present invention provides a semiconductor pressure sensor comprising:
a full-bridge circuit including semiconductor sensitive elements in a pressure sensitive structure and first and second output terminals; and
a filter for filtering output signals from said first and second output terminals, said filter including:
first and second resistors; and
a capacitor, one end of said capacitor being connected to said first output terminal through said first resistor, the other end of said capacitor being connected to said second output terminal through said second resistor.
According to the present invention, a second aspect of the present invention provides an exhaust system for motor vehicle comprising:
an engine;
particulate component removing means for removing particulate components in an exhaust gas from said engine; and
a semiconductor pressure sensor provided between said engine and said particulate component removing means for detecting a pressure of an exhaust gas from said engine before said particulate component removing means, comprising;
a full-bridge circuit including semiconductor sensitive elements in a pressure sensitive structure for receiving said pressure and first and second output terminals; and
a filter for filtering output signals from said first and second output terminals, said filter including:
first and second resistors; and
a capacitor, one end of said capacitor being connected to said first output terminal through said first resistor, the other end of said capacitor being connected to said second output terminal through said second resistor.
According to the present invention, a third aspect of the present invention provides the exhaust system based on the second aspect, wherein said filter removes a pulsating component in said exhaust gas from said engine.
According to the present invention, a fourth aspect of the present invention provides the exhaust system based on the second aspect, wherein a time constant of said filter is determined in accordance with a pulsating frequency of said exhaust gas from said engine.